In urban area, past industrialisation projects have contaminated the soil in many zones. Some of these zones are highly polluted by mixed contaminants. The terms mixed contaminants refer to two general components: the organic contaminants and the inorganic contaminants. The organic contaminants are usually water-insoluble and adsorbed on the surface of mineral grains or solids. The organic contaminants are often concentrated in the fine grain-size fraction of the material (“fines”). The inorganic contaminants, which include among others: arsenic, copper, mercury, selenium, zinc are found in the soil as: metal complexes adsorbed on the surface of minerals grains, mineral phases carrying the contaminants, metals alloys and metallic debris. According to various literature reviews (US-EPA, 1994; WASTECH, 1993), the inorganic contaminants are also largely confined in the fines. Because many of these polluted zones are currently being redeveloped by estate agency, the contaminated soil must be dealt with. Usually, the contaminated soil is excavated and disposed in a regulated landfill or decontaminated. However, for mixed contaminants, the number of decontamination processes available is limited. The available processes include: vitrification ex situ or in situ, soil washing, stabilisation/solidification and electro-remediation (for review, see US-EPA Internet site: www.clu-in.com). With the exception of soil washing and perhaps stabilisation/solidification the applications of these processes are restricted by their high costs.
Many commercial decontamination technologies for mixed contaminants operate on soil washing principles. All of these processes envision the soil contaminants as residing in the fines. Hence, the fines are isolated for the coarse fraction and submitted to different treatments using specially adapted washing fluids and froth flotation to recover contaminants from solids or from the washing solution. The coarse fraction is often treated by attrition scrubbing to remove the adsorbed fines. The latter being redirected to the fines treatment circuit.
In Canada, Tallon Technology, Environment Canada, technological fact sheet F1-04-95, Tallon Technology reports a soil washing process for mixed contaminants where a preliminary straightforward treatment involving washing, separation by particle size and magnetic separation recovers contaminants in the coarse fraction. A hydrometallurgical process treats the fines, rich in contaminants.
According to its final report, Pilot project report for the treatment of contaminated properties in the City of Montreal, CINTEC-ART has operated a soil washing pilot plant targeting the decontamination of soil from the Montreal area. Basically, the sand fraction was submitted to froth flotation while the coarse and fine fractions were separated by screening and hydrocycloning. The coarse fraction was used as backfill while the fines were routed towards a specialised landfill at high cost. The results were not conclusive and the project was eventually abandoned.
U.S. Pat. No. 5,268,128 teaches the treatment of contaminated particulate material were the material is first washed with a suitable contaminant mobilising solution. The coarse fraction, typically larger than 5 mm, is mechanically separated and returned to the site as backfill. The intermediate size fraction is abraded in an attrition scrubber for liberation of the fines. The contaminants dissolved from the particulate matter in the washing solution are adequately precipitated, concentrated and disposed.
U.S. Pat. No. 4,923,125 teaches a process for the purification of soil contaminated solely by organic material. Scrubbing, attrition and classification isolate the slow settling highly contaminated fines. The coarse fraction is treated by froth flotation for the removal of residual organic contaminants.
WASTECH, a U.S. multiorganization cooperative project, has reviewed available soil washing techniques (WASTECH, Soil washing, soil flushing, Innovative site remediation technology, 1993). The process used by Harbauer GMBH of Germany employs blade washers to blast off contaminants from sands and gravel fractions. The contaminants from the fines are then dissolved in the process water by a chemical extraction. The water is later treated by flocculation and coagulation. The U.S. EPA mobile soil washing system (MSWS) consists in a series of screens, hydrocyclones and froth flotation cells that isolate contaminated fines and a clean soil fraction. Waste-Tech Services Inc., has developed a similar technology based on froth flotation. The contaminants are collected in the froth while the cleaned soil is obtained from the flotation underflow. The Deconterra process is more complex. Basically the contaminated soil is separated in three fractions. A fraction smaller than 63 μm is not treated and routed towards the contaminated concentrate obtained from the process. An intermediate grain-size fraction is decontaminated by froth flotation and a coarse fraction by jigging.
From all of these descriptions, it is apparent that existing technologies are directed to the treatment of fines without or with only preliminary treatments of the coarse fractions.
Investigations of the contaminants distribution versus grain-size fractions in soils of certain areas have shown that the contaminants are not restricted in the fines, they are instead distributed in all grain-size fractions (FIG. 1).
FIG. 1 graphically illustrates the distribution of contaminants in contaminated soil from the site St-Ambroise/St-Paul in the City of Montreal according to grain size. It shows that the contaminants are limited to two main zones in that soil. A first zone below 12 μm contains 10 to 25% of contaminants. A second zone, corresponding to the grain-size interval from plus 38 μm to minus 11 mm, includes the prime portion of contaminants. The previously described technologies are not appropriate for treating this heterogeneous soil.
There thus remains a need to develop an effective treatment method for heterogeneous soil where contaminants are dispersed in fines, intermediate and coarse fractions.
It is therefore an object of the present invention to provide an improved method for treating the fines, intermediate and coarse fractions of contaminated soil.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art.